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You can do an OHM test on it but that might not tell you much if you don't know what the ohm reading should be.
Be sure that the sheild (on top, if it has one)there is no rust about to contact the HSI. Check the glow is uniform across the whole coil. Micro fractures can usually be seen then.
What ever you, DON"T touch the old one or the possible replacement with with your fingers.

I was considering brushing some ash or whatever it is from the HSI, but decided NOTo.uch it because skin oil would get on it.
Glow appears uniform but will have to look closer for fissures.
Yes, don't knowhathe resistance isupposed to be. Could call Rheem.
Thanks.

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i have a Peerless boiler for 21 years and i had to replace the hot surface ignitor about 6 times ,some lasted 2 years some lasted 5 . after the first one would not light on one of the coldest nights with it snowing i could not even get to the shop to see if we had one there . so what i had to do to keep the temp in the house was to hold a torch where the ignitor and waited for the gas valve to open and light the burner ,it worked .
so the next day i got two of them and always had an extra on hand

Comment

heat call throws power to the HSI, then the gas valve after the requisite time period, gas flows for a brief period, and if the flame sensor " sort of like a thermocouple" doesn't detect flame it kills the voltage to the gas valve.

For our Rheem "Criterion" furnace I have a spare squirrel-cage blower which I could make work temporarily.The limit switches I could wire around until replaced.

Be careful with this stuff. Blowers, belts and the like are no problem to replace etc. But when you start playing with failed limit switches and the like, it is not enough to just replace the switch. It is a primary safety device and it probably will not fail just because it is faulty. There may be issues with the venting, a possible plugged or otherwise faulty exchanger, air flow issues, too high temp. rise. Lots of stuff to start checking for to find the root cause of a limit switch failing, and bypassing it can lead to problems. If your house burns down with the limit bypassed, be sure that your insurance will not take care of you.
Just be careful with this stuff is all I am saying.

heat call throws power to the HSI, then the gas valve after the requisite time period, gas flows for a brief period, and if the flame sensor " sort of like a thermocouple" doesn't detect flame it kills the voltage to the gas valve.

To be more precise something called flame rectification is used to sense the flame. The controller applies an AC to the flame sensor. This passes from the sensor to the flame and to ground. The physics of the sensor and the flame act like a diode and rectifies the current (I think probably half wave rectification). This is sensed by the controller to prove the flame.

... something called flame rectification ... The physics of the sensor and the flame act like a diode and rectifies the current (I think probably half wave rectification). This is sensed by the controller to prove the flame. ...

Interesting.
What voltage is needed to pass through the flame to circuit ground?
Yes, that would be halfwave.

I almost suffered flame rectification when I passed gas near a campfire.

Interesting.
What voltage is needed to pass through the flame to circuit ground?
Yes, that would be halfwave.

I almost suffered flame rectification when I passed gas near a campfire.

Not sure about the voltage but the actual current is very small. There are many ways to detect a small current which is heavily submerged in noise which I think would be the case here. One method which comes to mind is where you mix the signal with a synchronous version of the original stimulus signal and then use a very narrowband low pass filter to filter out the aliases and leave only the dc component. This is one way to do it of course - I'm sure there are others.

... mix the signal with a synchronous version of the original stimulus signal and then use a very narrowband low pass filter to filter out the aliases and leave only the dc component. This is one way to do it of course - I'm sure there are others.

I'm curious why AC would not pass through the flame plasma.
Mustry an experiment with one of my high voltage transformers and a neon tube.

To be more precise something called flame rectification is used to sense the flame. The controller applies an AC to the flame sensor. This passes from the sensor to the flame and to ground. The physics of the sensor and the flame act like a diode and rectifies the current (I think probably half wave rectification). This is sensed by the controller to prove the flame.

No flame = no current.

Blue can could you explain what happens when the flame sensor is shorted out. ie. if it is touching the burner. I don't understand why a short wouldn't tell the control it has flame. This has baffled me since the 1st time I pulled all the burned up screen material from an A.O. smith water heaters flame sensor.

Does a residential water heater have a flame sensor?
Thought it had only a standing pilot light and thermocouple, otherwise it would need to have power to operate the flame-sensing circuitry. (I wish it did so that I could install an electric damper to stop all the heat loss up the vent.)

A circuit couldetect AC and know it is a short and not DC caused by flame conduction.

Blue can could you explain what happens when the flame sensor is shorted out. ie. if it is touching the burner. I don't understand why a short wouldn't tell the control it has flame. This has baffled me since the 1st time I pulled all the burned up screen material from an A.O. smith water heaters flame sensor.

The flame sensing in water heaters is different to that in a furnace (or for that matter tankless with a non-standing pilot). The principle of the thermocouple is that the junction of two dissimilar metals when heated will produce a voltage and that voltage is used to keep the gas valve open for the pilot.

If the thermocouple is shorted to the burner it will not be able to develop a potential difference since the current will flow to the burner (and probably to ground) so yes it will stop working.